High pressure, high temperature piston-cylinder apparatus are used for a variety of purposes and, specifically, for the production of diamond crystals. In such apparatus, a core of charge material; in the case of diamonds, graphite and a carbon solvent metal; is confined within a reaction chamber in the form of a cylinder and is heated and subjected to pressure therein.
Between the cylinder and core of charge material, there is placed a surrounding sleeve of electrical insulating material, and the charge is heated by passing an electric current therethrough. The sleeve inhibits the transfer of heat outward from the core to the cylinder and surrounding apparatus and confines the heating current to the core.
The insulating material is generally cylindrical, fitting closely within the inside diameter of the cylinder and extending to near the ends of the cylinder. The core of charge material is usually tightly fitted within the insulation material, and may also be cylindrical in shape. One end of the cylinder is closed and the piston fits in the open end of the cylinder and is advanceable into the cylinder in the direction toward the closed end of the cylinder. The inside diameter of the piston cylinder and the outside diameter of the piston that fits into the cylinder usually form a snug sliding fit respectively with each other.
With the core of charge material placed in the cylinder and the insulation and piston in place, electrical heating of the core of charge material takes place by passing electrical current from the closed end of the cylinder, through the core of charge material and on through the piston, thereby heating the core of charge material to any desired temperature upon command by the operator. Pressure in the cylinder is increased by advancing the piston into the cylinder at a controlled rate and with a known force exerted on the piston.
In the manufacture of diamonds, it is not only extremely important to know the precise temperature and pressure conditions on the core of charge material, but also to insure that the temperature and pressure conditions are constant throughout the entire core of charge material. It is known in the industry that if a core of graphite and carbon solvent are placed in the cylinder and subjected to sufficient pressure and temperature, diamond crystals will result. The size and quality of the diamond crystals can vary depending upon how well and uniformly the growth conditions of temperature and pressure are held within the cylinder.
One of the problems associated with the growth of diamonds in a piston-cylinder apparatus is the problem of compensation for the volume change that takes place as a result of the conversion of carbon from graphite form to diamond form. There is a significant difference in the density of diamond and the density of graphite. The density of diamond is about 3.5 while the density of graphite is only about 2.25.
Thus, when the sufficient temperature and pressure conditions are present and the graphite begins converting into diamond, there is a corresponding reduction of volume in the core of charge material. As mentioned earlier, since the pressure must be kept constant, then the piston has to be advanced into the cylinder as the volume reduction is occurring.
In all prior art machines, the insulating material is located between the inside diameter of the cylinder and the core of charge material. The insulating material is ideally of constant cross section throughout its length to maintain uniform current density through the charge and has uniform wall thickness such that no temperature gradients build up inside or along the core of charge material.
If the above conditions are not maintained during the complete cycle of the process, the relative temperature in the core would vary and "cold" or "hot" spots will build up in the core of charge materials during the reaction and thereby hinder the quality of diamond crystals produced by the fact that the temperature is not held at a known value for a given period of time.
In these prior art machines, and by the very nature of the diamond crystals forming from graphite, the piston must advance into the cylinder. As the piston advances into the cylinder, since it is a snug sliding fit with the inside diameter of the cylinder, it necessarily begins to crush the insulating liner. The radially outward edges of the piston contact the insulating material and begin to buckle or compress it along with the core material.
When the buckling or compression of the insulating material occurs, uniform current density is not maintained through the core of charge material and the wall thickness of the insulating material between the core of charge material and the cylinder wall is not uniform. When the uniform conditions are not maintained, hot and cold spots now develop throughout the core of charge material. This effect is necessarily detrimental to the controlled formation of uniform size and quality diamonds.
In the known "belt" type high pressure apparatus, the pistons taper inwardly toward the charge and the ends of the cylinder in which the charge is disposed are correspondingly tapered. Gaskets formed of deformable electrical insulating material, such as pyrophyllite, are disposed between the tapered pistons and the tapered ends of the cylinder and seal the charge in the cylinder.
The gaskets must deform to permit the piston to advance into the cylinder to compact the charge, and the tapered configuration of the pistons and cylinder ends present a substantial area which absorbs load from the pistons while limiting the amount the pistons can advance into the cylinder.